Story importance

Unknown

I've mentioned this elsewhere, but wanted to make a finer point of it:

If the story being told is important, it needs to be available to everyone, even the newest player.

That means low level enemies (with commensurate rewards, sure). Maybe they can ramp up on replays, but if the Marvel IP and its story are important, and that's a big if, it can't be tucked behind teams of level 230 enemies and only offered for a week or so.

Unknown

Mabey they are going to add the pve episodes alongside the prologue permanently later in the game so that people can play whenever they want to play through the storyline and it doesn't have to be high level fights as it gives the same rewards as prologue.

Unknown

It is a F2P game with an expensive IP. There is plenty of reason for them to want you to spend money to see the whole game. That being said, you most certainly don't need to pay. Really, it's the least punishing F2P I've ever played. There isn't much of a wall in the prologue. But as posted before, I hope you are right and they decide to make the episodes playable in the future in some static form. That would allow newer / free players to hear the story at their own pace.